Iowa - Significance region: Local
| Noble-Kendall House |
| Norman Apartment Building |
| North Fellows Historic District |
| North Grand Avenue Residential Historic District |
| North Grinnell Historic District |
| North Presbyterian Church |
| North River Stone Schoolhouse |
| Northwest Davenport Savings Bank |
| Northwest Davenport Turner Society Hall |
| Northwestern Hotel |
| Northwood Central Avenue Historic District |
| Norton, Charles Henry and Charlotte, House |
| Notbohm Mill Archaeological District |
| Oak Hill Cemetery Historic District |
| Oak Hill Historic District |
| Oakland Cemetery |
| Oaklands, The, Historic District |
| Oak Lane Historic District |
| O'Brien County Courthouse |
| Ockershausen, Henry, House |
| Odd Fellows Block |
| Odd Fellows Hall |
| Odd Fellows Hall |
| Odenweller, F. F.-James P. and Nettie Morey House |
| Ogburn, William, House |
| Old Allamakee County Courthouse |
| Old Chapel Hall |
| Old City Hall |
| Old East Paint Creek Lutheran Church |
| Old Fourth Ward Southeast Historic District |
| Old Harrison County Courthouse |
| Old Jail |
| Old Lenox College |
| Old Main Historic District |
| Old Mill House |
| Old Post Office |
| Old Settlers' Association of Johnson County Cabins |
| Old Settlers' Association Park and Rhodham Bonnifield House |
| Old Town Historic District |
| Old Worth County Courthouse |
| Onawa IOOF Opera House |
| Organ, Alexander, House |
| Orpheum Theatre and Site |
| Osage Commercial Historic District |
| Osborne, Edmund B., House |
| Osceola Commercial Historic District |
| Osceola County Courthouse |
| Osceola Masonic Block |
| Oskaloosa City Hall |
| Oskaloosa City Park and Band Stand |
| Oskaloosa City Square Commercial Historic District |
| Oskaloosa Fire Station |
| Oskaloosa Monthly Meeting of Friends Parsonage |
| Oskaloosa Post Office |
| Ossian Opera House |
| Ottumwa Cemetery Historic District |
| Ottumwa Public Library |
| Ottumwa Young Women's Christian Association |
| Our Mother of Sorrows Grotto Historic District |
| Our Savior’s Lutheran Church |
| Outing Club |
| Overland Waterloo Company Building |
| Oversen, Julius and Anine House |
| Owl's Head Historic District |
| Oxford Commercial Historic District |
| Page County Courthouse |
| Pahl, Henry, House |
| Paine, Samuel and Mercy, House |
| Painter-Bernatz Mill |
| Palmer, B. J., House |
| Palmyra Methodist Episcopal Church |
| Paradise Block Historic District |
| Paradise Farm |
| Paramount Theater Building |
| Parker's Opera House |
| Parker's Woods Park Historic District |
| Park/Glen Avenues Historic District |
| Park Hotel |
| Park House Hotel |
| Park Motel |
| Park-to-Park Residential Historic District |
| Paul-Helen Building |
| Paulsen, Peter J., House |
| Paustian, Henry, House |
| Payne, O. E., House |
| Peak, George B., House |
| Pease, Dr. Luther L. and Susette E. (Baker), House |
| Peirce, John, House |
| Pella High School |
| Pella Opera House |
| Peoples Nationals Bank |
| Perham House |
| Perkins, Charles W. and Nellie, House |
| Perkins, George and Lola, House |
| Perrigo-Holmes House |
| Perry and Brainard Block |
| Perry Carnegie Library Building |
| Perry, T. B., House |
| Peters' Barber Shop |
| Petersen, Max, House |
Data from United States Park Service and Wikidata
